Peer Reviewed: Mid-IR Fiber-Optic Sensors - ACS Publications

Mid-IR Fiber-Optic

SENS

RS

Boris Mizaikoff Georgia Institute of Technology

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A N A LY T I C A L C H E M I S T R Y / J U N E 1 , 2 0 0 3

Chemical IR sensors can provide continuous information on the probed environment and are on the verge of taking the significant step from experiment to real-world applications.

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occasionally assays the environment. (IR fiber-optic devices can also directly sense physical parameters, such as temperature, but those schemes are not discussed here.) Modern chemical sensor technology is on the verge of taking the significant step from the experimental stage to real-world applications (3). Optical sensor technology is characterized by its versatility and potential for miniaturization (4). This has been aided by fiber-optic waveguide technology originating in the telecommunications industry, which has helped convert benchtop-style optical instruments to compact optical sensors and sensor systems (5).

Setting the stage Spectroscopic sensors capable of simultaneously detecting and discriminating multiple analytes face new challenges because they are now called upon to determine chemical and biological agents for homeland

©2003 DANIEL PECK STUDIOS

mong optical sensing schemes, mid-IR fiber-optic sensors are gaining attention because of their inherent molecular selectivity. Fiber-optic materials transparent in the mid-IR spectral region up to wavelengths of 20 µm offer access to fundamental vibrational and rotational fingerprint absorptions of organic molecules. In addition, increasingly available midIR optical waveguides— mainly based on materials such as heavy metal fluorides, chalcogenides, silver halides, tellurium halides, sapphire, and hollow waveguide structures— enable environmental analysis, process monitoring and control, and, most recently, applications in the biological and biomedical fields (1, 2). This article focuses on chemical IR sensors and sensor systems, which can provide continuous information on the probed environment. This is a different vision than is found in other chemical-sensing systems, which use the sensor merely as a detector that

J U N E 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y

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ical responses into data that are compatible with standard comAttenuation Transmission IR fiber-optic waveguides Material (dB/m at 2.94 µm) (µm) munication protocols so that Hollow waveguides 2–19 0.5 (silica) chemical sensors can be networked. Silver halides 4–18 3 Broadband IR, UV, or Tellurium halides 3–13 0.5 Raman sensors are generally capable of multicomponent Chalcogenides 2–11 5 analysis because they generate signals with inherent molecuFluoride 0.25–4.5 0.08 lar specificity. However, the Sapphire 0.5–3.1 0.4 difficulty of interfacing a spectral response with standardLow-OH SiO2 0.3–2.5 0.012 at 0.82 µm ized data communication pro0.25–1.4 SiO2 0.0004 at 1.32 µm tocols increases with the 0 2 4 6 8 10 12 14 16 18 20 µm complexity of molecular signatures that frequently overlap. In other words, independFIGURE 1. The most commonly used mid-IR transparent fiber-optic materials and their releently operated IR sensors vant properties for fiber-optic sensing. Transmission and attenuation data are the average require automated interpretaof reported values. tion and evaluation of broadband mid-IR spectra. This task includes deconvoluting overlapping responses from multiple components, correcting for drift, and presenting quantitadefense, security of public and restricted spaces, trans- tive results on individual analytes. This problem is currently portation safety, and the biomedical and health fields. being tackled from two sides: higher-order sensors based on orSensors that measure molecular fingerprints can iden- thogonal transduction schemes integrated into one device (6) tify and quantify adverse species, if they can sufficient- and the application of multivariate chemometric calibration and ly discriminate these target analytes from the ambient data evaluation techniques for reliable evaluation of complex sensor responses (7 ). matrix and interferences. The development of low-cost spectroscopic sensing devices However, the transducer or sensor is only one of many components that must be interfaced with a currently appears to be difficult because of the fundamental monitoring, early warning, security, or rapid-response technological and engineering requirements. Despite its roots scheme. Smart networking of sensor signals and data in chemistry, chemical sensor technology is a highly interdiscievaluation provides a coordinated and timely response plinary research field, harvesting advances from materials scito a multitude of transducer signals that report on ences, information technology, mechanical, electrical and optichanges of the physical (e.g., temperature, pressure, cal engineering, device physics, and (bio)molecular recognition humidity) and chemical (e.g., volatile organic com- studies. Advances in microfabrication, miniaturization of sensor pounds, cells, spores) environments. Hence, the fun- components—including light sources (e.g., quantum cascade damental visions of this concept are networked, lasers), detectors (e.g., quantum well IR photoconductive detechigher-order sensors that provide orthogonal infor- tors [QWIPs]), and integrated optics (e.g., on-chip waveguides, mation; are capable of wireless communication; can photonic bandgap materials)—in concert with optimized molecoperate with little maintenance, long lifetimes, and ular recognition membranes (e.g., molecularly imprinted polyminimum power requirements; and offer sufficient mers) should solve some of the problems and offer a promising future for spectroscopic sensors in modern analytical chemistry. robustness, sensitivity, and selectivity. Many of these operational parameters are already realized for most physical transducers, but what about IR fiber-optic materials future needs? For example, long-term stability of the IR optical fibers are defined as waveguides transmitting radia(bio)chemical sensing interface and related sensor tion at wavelengths >~2 µm. They are divided into three main fouling is a major concern. The solutions to these categories, according to their material properties: glass fibers problems are not imminent. Another problem is the (e.g., chalcogenides and fluorides), polycrystalline (e.g., silver efficient “on-board” translation of broadband chem- halides) or crystalline fibers (e.g., sapphire), and hollow wave260 A


guides (e.g., hollow silica or sapphire tubes). Properties for the most relevant IR fiber-optic materials are summarized in Figure 1. Dynamic progress in this field, nurtured by the demands from telecommunications for far-IR (>20 µm) transparent materials suitable for long-wavelength optical communications, has been discussed elsewhere (8). Non-silica-based IR transparent fibers made from arsenic trisulfide first appeared in the 1960s (9). It was not until the early 1970s that single-crystal fibers were reported (10), and a wide variety of new IR transparent waveguides shortly followed, growing out of military applications and the need to deliver surgical CO2 laser radiation. Despite sapphire fibers’ advantageous chemical and mechanical properties, only a few sensing applications incorporating them have been reported, such as the measurement of gaseous hydrocarbons at high temperatures and cycle-resolved vehicle engine emission monitoring (11, 12). Fluoride fibers are characterized by low attenuation losses (13), and applications, such as investigating the diffusion of propane into a Teflon cladding, have been reported (14). However, the mid-IR transmission window for fluoride fibers is limited, and they are susceptible to moisture. Chalcogenide glasses are considered the technically most advanced IR fiber-optic waveguides and have been used for remote hydrocarbon gas sensing and determining organic solutes in water (15–17 ). Tellurium halide fibers are highly resistant to corrosion and degradation and have been used to determine alcohol in water (18, 19). Silver halide fibers are among the most promising materials for fiber-optic IR sensors, providing access to the whole spectral range of interest (3–18 µm) (20). Because of the versatility of the fibers, numerous sensing applications for the detection of organic compounds have been reported (1, 2). Structural tubes made from dielectrics, metals, or metal-coated materials provide hollow waveguide structures that act like an optical fiber with an air core (21). IR radiation is guided by reflection on the inside walls. Hollow waveguides acting as a capillary flow cell enable gas-sensing applications for numerous relevant compounds. This miniaturized gas-sensing approach, in combination with quantum cascade lasers, has recently been demonstrated for the first time (22).

tures of analytes are investigated after membrane extraction or membrane enrichment within the probed analytical volume. Ideally, the sensing membrane is directly coated onto the transducer surface. Frequently, such systems are classified as physicochemical sensors. Most mid-IR sensing schemes are based on the fundamental principle of internal reflection spectroscopy or, more specifically, attenuated total reflection (ATR) (23, 24). Total internal reflection of electromagnetic radiation occurs when light at an angle of incidence greater than a critical angle c is reflected at the interface between the optically denser waveguide (n1) and the adjacent optical thinner medium (n2), such as an analyte-enriched polymer membrane coated onto the transducer surface. In this situation, c is equal to the arcsine n2/n1. This results in part of the electromagnetic radiation propagating along the waveguide surface and leaking into the contiguous environment. Such externally guided radiation is called evanescent wave or evanescent field. The evanescent field penetrates with an exponentially decaying field amplitude into the adjacent medium and interacts with molecular species present within the probed analytical volume. Figure 2a schematically shows a section of a fiber-optic waveguide (yellow) with refractive index n1 coated with an enrichment membrane (green) of refractive index n2, with n1 > n2. At a given wavelength , the penetration depth of the evanescent field dp can nominally be calculated as

Chemical sensor

technology is a highly interdisciplinary research field.

IR optical sensing principles Fiber-optic sensing schemes are divided into two main groups: direct sensors, which detect changes in optical properties or spectral characteristics, and indirect sensors or indicator-based sensors, which use various types of chemical recognition processes to translate chemical signals at the interface of the sample into changes of analyte-specific optical signatures. Although the separation between these categories is somewhat fuzzy, we will focus on indirect chemical IR sensors. Characteristic spectral IR fea-

dp =

2π

n 12 sin2–n 22

As indicated in Figure 2a for aqueous-phase measurements, the thickness of the membrane is selected to be larger than the maximum nominal penetration depth of the evanescent field in the spectral region of interest. Using hydrophobic polymers for analyte enrichment will minimize the presence of interfering water within dp for IR chemical sensors. The interaction of the evanescent field with enriched molecular or ionic species provides IR spectra similar to conventional absorption measurements recorded as transmissions. The evanescent field propagates at the waveguide/membrane interface, which results in minimal disturbances from adverse physical properties of the sample matrix, such as turbidity (25). ATR spectroscopy in the mid-IR spectral range has become a well-established laboratory technique that uses conventional crystalline ATR elements made from materials such as zinc selenide, germanium, or silicon in the form of a prism, trapezoid, rod, or semisphere (26). Introducing mid-IR transparent fiber optics extends this concept into the realm of optical sensor J U N E 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y

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cost to be adapted to the needs of the analyst—multiwavelength (broadband) IR sensors (a) for multicomponent analysis or monochromatic (narrowband) FTIR MCT IR sensors for high-sensitivity target analysis. Commonly, IR Quantum cascade laser DTGS fiber-optic sensors are combined with FTIR spectromeTunable lead salt laser QWIP ters, thereby providing multicomponent analysis and a high OPO Thermopile degree of flexibility. Miniaturized sensors for target analysis Blackbody Pyroelectric use tunable lead salt laser diodes or, more recently, quan(b) tum cascade lasers. Optical parametric oscillators (OPOs) have gained importance as mid-IR light sources as their FIGURE 2. (a) An evanescent field sensing for liquid-phase analysis and (b) a hollow physical dimensions have dewaveguide sensing for gas-phase analysis, including the most commonly applied light creased. Although light-emitsources and detectors. ting diodes in the mid-IR range are commercially available, their use is limited by their low radiative energy output. At high analyte concentration levels, the combinatechnology called fiber-optic evanescent wave sens- tion of blackbody radiators with wavelength-selecting bandpass filters certainly represents a cost-efficient solution. The selection ing (FEWS) (27 ). IR gas analyzer schemes rely mostly on a trans- of appropriate detection schemes is among the crucial paramemission cell with an IR light source and detector ters that determine the achievable limit of detection. Broadband semiconductor detectors, such as liquid N2connected by appropriate coupling optics. Most recently, gas-sensing concepts have taken advan- cooled mercury–cadmium–telluride (MCT) systems, are most tage of hollow waveguides simultaneously acting commonly used for high-sensitivity applications. Various detecas miniaturized capillary gas cells and fiber-optic tor materials with broad- or narrowband response are available IR waveguides. Figure 2b schematically illustrates throughout the entire mid-IR spectral range. Field-deployable that radiation transported inside the waveguide by IR sensors usually rely on thermoelectrically cooled MCTs or reflection at the walls (dark gray) interacts with room-temperature-operated deuterated triglycine sulfate degaseous analytes inside the hollow core of the tectors (DTGSs). If cost is of no concern and the highest senfiber. Provided there are appropriate gas inlet/out- sitivity for a field application is required, semiconductor deteclet modules, volatile analytes present in the gas tor elements are contained within miniaturized, closed-cycle phase or after liquid/gas extraction are readily de- Stirling coolers, providing temperatures as low as 77 K in a detected inside the hollow waveguide capillary (Fig- tector package that measures ~10 cm3. Less expensive detector ure 3b) (28, 29). Another strategy for detecting solutions for higher concentration ranges are based on pyrovolatile organic compounds in water is based on electric materials or thermopiles. Microfabricated wavelengthpre-enrichment from the sample solution directly selective detection devices based on QWIPs or microbolometers flowing through an internally polymer-coated hol- are of increasing importance in combination with monochrolow waveguide (30, 31). matic light sources. Although direct sensing by immersing ATR-based fiberoptic sensors into a sample solution is feasible (32), the sensiIR sensor technology The fundamental IR sensing principles discussed tivity is usually limited to the high parts-per-million concentraearlier can be combined with an extensive selection tion range by the interfering absorptions of water (O–H of light sources and detection schemes (Figure 2). stretching vibration 1,3 at 3300 cm–1; O–H bending vibration This allows spectral selectivity, sensitivity, and system 2 at 1640 cm–1; combination vibration 2+L at 2100 cm–1; 262 A


and libration vibration L at 750 cm–1). Analysis at low partsper-million and -billion concentration levels usually requires exposing the sample to hydrophobic chemical recognition membranes, which minimizes the water background and enriches the species of interest within the volume probed by the evanescent field (Figure 3a). Hydrophobic polymer layers (e.g., polyisobutylene, ethylene– propylene copolymer, Teflon AF) are coated onto the actively transducing waveguide surface (33–35), following the general concept of solid-phase microextraction. Hence, water is widely excluded from the analytical volume probed by the evanescent field. For example, water/polymer partition coefficients of volatile chlorinated hydrocarbons range between 100 and 1000 (36). The selection of appropriate enrichment membranes is important for optimizing the limit of detection, whereas the coating thickness mainly influences the sensor response time. As recently modeled, the response time is substantially affected by the diffusion behav(a) ior of analytes in the water Liquid-phase sample column. Higher flow velocities and diffusivity, which can be supported by an optimized flow-cell design, result in quickly reaching equilibrium IR light source conditions (37 ). Table 1 gives a selection of quantitative data (b) for various analytes and memLiquid-phase branes. Although this list is by sample no means exhaustive, detection limits for this diverse group of analytes usually extend well IR light source below the milligram-per-liter concentration range.

Environmental applications

strategies beyond the classical laboratory analysis. The daily use of ~70,000 synthetic chemicals worldwide and the more than 700 different organic constituents that may be prevalent in drinking water illustrate the complexity of this task and present an enormous challenge to in situ sensors. Sophisticated analytical methods based on miniaturized separation techniques and species detection can offer discontinuous analysis at the trace and ultratrace concentration level. Sensors are a complementary technology, offering in-field surveillance, threshold monitoring, and rapid screening abilities. Future analytical strategies for water quality assessment will combine sensors with classical analysis into a comprehensive network of analytical tools. Detection of volatile aliphatic, aromatic, and/or halogenated hydrocarbons in water using IR fiber-optic chemical sensors

Extraction/enrichment membrane IR fiber

Detector

Gas-phase sample

Extraction/enrichment module

Hollow waveguide

Detector

FIGURE 3. (a) An evanescent field chemical sensor with a chemical recognition/ extraction/enrichment interface and (b) a hollow waveguide sensor for analysis of dissolved volatile compounds by a liquid–gas extraction module or direct gas-phase sensing.

Based on the number of publications, environmentally relevant compounds are the primary applications for these sensors, followed by process analysis and biomedicine. Analysis of volatile organic pollutants, such as chlorinated or aromatic hydrocarbons, is the area that will probably have the most widespread application of mid-IR optical systems. Selected examples focusing on water analysis highlight the potential of these sensors. Environmental problems, such as the limited availability of high-quality drinking water, preservation of marine ecosystems, and an increasing number of global water pollution problems, such as endocrine-disrupting compounds, demand analytical

has been intensively investigated recently (2). Most devices transmit IR radiation from an FTIR spectrometer into mid-IR transparent fibers that act as both waveguide and chemically modified active transducer. Silver halide fibers are particularly suitable, providing optical transmission up to a wavelength of 18 µm, which is well into the fingerprint regime characterized by highly selective absorption patterns for many organic compounds. An entirely novel field of application for IR spectroscopic J U N E 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y

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Table 1. Limits of detection for a variety of analytes and polymer membranes. Analyte

Membrane

Benzene

Poly(styrene-co-butadiene)

Chlorobenzene


Dibutylphthalate

Poly(acrylonitrile-co-butadiene)

1,2-Dichlorobenzene

Ethylene/propylene copolymer

1,2-Dichloroethylene


2,4-Dichlorophenol


2,4-Dichlorophenoxyacetic acid

Molecularly imprinted polymer

Diethylphthalate


Dimethylphthalate


Ethylbenzene


m-Creosol


o-Xylene


p-Bromophenol


p-Chlorophenol


Tetrachloroethylene


Toluene


1,2,4-Trichlorobenzene


Trichloroethylene


Trichloromethane


sensing systems is in the subsea environment. The first marine FTIR sensing system was demonstrated with the multicomponent analysis of chlorinated aliphatic (tetrachloroethylene at ~50 ppb) and aromatic hydrocarbons (mixture of benzene, toluene, and o-, m-, and p-xylenes at ~200 ppb) in seawater during extensive tests in flume tank facilities (25, 38, 39).

Making the applications work Figure 4a shows a 3-D scheme of a fiber-optic sensor head with dimensions suitable for groundwater analysis via monitoring wells (10 min, the equi300 librium response can be approximated by evaluating the slope of the response 100 curve after 1–3 min. However, this data evaluation strat200 egy requires profound knowledge of 600 the sensor response and stability, which are difficult to maintain during field de300 ployment of diffusion-based sensing sys100 tems. Recent field experiments show sufficient robustness of the developed 200 IR chemical sensor head for groundwa1000 ter monitoring during extended application periods (41). However, auto500 nomous long-term operation requires compensation for baseline drifts that 500 occur due to membrane delamination 400 or polymer aging. Chemometric data evaluation techniques provide the fun500 damental basis to tackle these problems. 500 Novel algorithms derived from principal component regression have been 80 developed to automatically compensate 600 for baseline drifts as an integrated part of chemometric data evaluation schemes 500 (42). Baseline drifts are modeled by 200 polynoms orthogonal to the principal components describing the concentra900 tion result. Alternatively, drift components are modeled by synthetic pseudoprincipal components along with the conventional principal components characterizing the analyte peaks. Successful tests with synthetic spectra and real-world mid-IR chemical sensor data indicate that both algorithms are a substantial improvement for sensors in the field. Furthermore, a chemometric algorithm for automated recognition of noncalibrated absorbers was developed and effectively tested for application in optical spectroscopy and chemical sensing (7, 43). Although chemometric data evaluation increases the reliability of spectroscopic sensor readings, the robustness and lifetime of the chemical-sensing interface are still critical. Despite the inherent robustness of most conventional aliphatic polymers, such as ethylene/propylene copolymer, polyisobutylene, and low-density polyethylene, aging effects, swelling, and occasional delamination may occur with extended exposure to polluted aqueous phases. Sol–gel-processing techniques have successfully been applied to the deposition of thin porous films at surfaces of planar optical waveguides. However, researchers

Absorbance (a.u.)

have only recently investigated the suitability of sol–gels in the The future of IR sensors search for improved IR sensing membrane materials (44). The evolution of mid-IR sensor technology in the The sol–gel process facilitates preparation of glasses and ce- last decade has been characterized by achievements ramics at low temperatures by hydrolysis and polymerization of in miniaturization of optical components, light organic precursors. These materials have large surface areas; sources, and detectors (based on advances in mitunable porosities; and favorable optical, dielectric, thermal, crofabrication and microelectronics) and improveand acoustic properties, which make sol–gels a good choice as ments in the chemical recognition interface. In the an enrichment layer at a transducer surface. By varying the quest for improved miniaturized light sources, the preparation parameters, such as type of catalyst, pH, and H2O/Si molar ratio, the chemical properties of the resulting sol–gel membrane are tunable over a wide range. In particu(a) lar, the introduction of organically modified siloxanes (orFTIR MCT mosils) as precursors enables flexible control of membrane polarity and porosity and al(b) lows incorporation of reactive OAPM 2 OAPMs functional groups into the 0.05 sol–gel matrix. 0.30 TeCE 0.04 FTIR ATR spectroscopic 0.20 experiments using trapezoidal 0.03 40 min 0.10 germanium or ZnSe ATR ele0.02 0.00 ments coated with various 0 min 0 1000 2000 3000 sol–gel layers greatly suppress 0.01 Time (s) DCB TriCE interfering water absorption 0.00 while enriching organic analytes 1300 1200 1100 1000 900 (45–47). Recently, we demonWavenumber (cm ) strated that such sol–gel coatings are compatible with silver halide fibers (Figure 5) (48). FIGURE 4. (a) System for a chemical IR fiber-optic evanescent field dip sensor for groundThe active transducer region of water, effluent, and leachate analysis. (b) Multicomponent analysis of an aqueous sample –1 the fiber coated with the sol–gel containing 10 ppm (v/v) each of tetrachloroethylene (TeCE, 911 cm ), trichloroethylene –1 –1 membrane is located in a flow (TriCE, 932 cm ), and 1,2-dichlorobenzene (DCB, 1036 cm ). The membrane is an ethylene/ cell (Figure 5a). A scanning propylene copolymer; the sensing fiber is silver halide. The inset shows a typical enrichelectron microscopy image of ment curve for TeCE. the fiber at the intersection of sol–gel-coated and -noncoated fiber surface areas demonstrates homogeneous coverage, which is further confirmed by excellent suppression of water background absorptions (Figure 5b). most significant impact certainly derives from the Figure 5c shows reversible enrichment of a 50-ppm aqueous introduction of quantum cascade lasers (QCLs) in solution of parathion, a neurotoxic pesticide, into a sol–gel the mid-1990s (49). membrane prepared by a base-catalyzed approach. The current State-of-the-art QCLs provide single-mode limit of detection with this IR chemical-sensing approach is 2 light emission with milliwatts of output power in ppm. However, it is anticipated that optimizing the properties the mid-IR region and can be operated at room of the sol–gel layer will enable continuous measurements at temperature in continuous-wave mode (50). Inconcentrations of a few hundred parts-per-billion, while main- stead of conventional electron-hole recombination taining the unsurpassed robustness of the coating. Application across the bandgap of a semiconductor material, to various nitro aromatic compounds has also been demonstrated. QCLs are based on electronic intersubband transi–1

J U N E 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y

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Absorbance (a.u.)

Peak area (a.u.)

in Switzerland) is a roomtemperature-operated QCL directly pigtailed with a freestanding planar silver halide (a) waveguide with a thickness of

Peer Reviewed: Mid-IR Fiber-Optic Sensors - ACS Publications

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